Motor Starter Protection, Copper Conductor Ampacity, and more

CODE CITATIONS: Article 100-Definitions Article 210-Branch Circuits; Article 240-Overcurrent Protection; Article 250-Grounding; Article 310-Conductors for General Wiring; and Article 700-Emergency Systems Type "2" Motor Starter Protection Q: I am a maintenance electrician in a manufacturing plant. The other day, I overheard my boss talking to a sales engineer about Type 2 motor controller protection. Will you explain the meaning of Type 2 protection for a motor controller? A: Type 2 motor starter protection is described in IEC's publication 947-4-1. It generally states that under short-circuit conditions, the contactor or starter cannot cause any danger to persons or the installation, and must be suitable for further use. Contact welding is permitted, but the manufacturer must indicate how repairs are to be made before placing the starter back in service. Under Type 2 protection, no damage is allowed to the contactor or overload relays. However, light contact welding is permitted. Light contact welding means that the contacts can be easily separated. Type 1 protection cannot cause any danger to persons or the installation under short-circuit conditions, but internal damage may require repairs and parts replacement before the starter is returned to service. According to UL 508, the standard for Industrial Control Equipment, contact welding, and thermal overload relay burnout is permitted under some conditions during short circuit testing of a motor controller. A National Electrical Manufacturers Association (NEMA) publication with the title Type 2 Motor Protection Fuse Guide has tables for various voltages and motor horsepower ratings that show the type and fuse ampere rating that must be used to obtain Type 2 protection. For example, a 7 1/2 horsepower, 230 volt, three-phase motor controlled by a NEMA size 1 starter will have Type 2 protection where a 35 ampere Type J or Type RK1 fuse is used for the motor branch circuit short-circuit and ground-fault protection. Notice that this fuse ampere rating is lower than the value permitted by National Electrical Code (NEC) Table 430-152 for dual-element, time delay fuses because Section 430-52 (c)(1) Exception No. 1 allows a 40-ampere fuse for branch circuit short-circuit and ground-fault protection. No. 12 Copper Conductor Ampacity Q: My tool buddies and I enjoy challenging each other with your "Code Questions and Answers" article. Recently we discovered a mistake. On page 39 of the September 1999 issue of Electrical Contractor magazine, a question was presented concerning branch circuit correction factors. In the explanation, it was stated that a No. 12 AWG copper conductor with THWN insulation was rated for 25 amps. It is really rated for 20 amps. Would you comment on this? A: You and your tool buddies are generally correct in your statement that No. 12 copper wire is rated for 20-amperes. However, the ampacity of a No. 12 Type THWN copper conductor is listed in Table 310-16 at 25. Moreover, Ampacity is defined in Article 100 of the NEC as follows: "The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating." The ampere rating of the overcurrent device protecting the branch circuit determines the branch circuit rating. This information is found in Section 210-3. Although No. 12 copper wire can carry 25 amperes continuously where there are no more than three current-carrying conductors in a raceway or cable assembly, and the ambient temperature is 30 degrees C (86 degrees F). The asterisk that accompanies the numeral (12) in Table 310-16 refers to Section 240-3 for additional information. In Section 240-3(d) the restriction that limits the overcurrent protection for a No. 12 copper conductor to 20 amperes appears. However, this restriction does not apply to branch circuits that supply loads mentioned in Section 240-3(g). For example, a 1 1/2 horsepower, single phase, 120 volt motor may be supplied by Number 12 copper wire protected by a 50 ampere circuit breaker. Here are the calculations for this example. The full load current of a 1 1/2 horsepower, single phase, 120 volt motor is listed at 20 amperes in Table 430-148. The motor branch circuit conductor ampacity cannot be less than 125 percent of motor full load current to comply with Section 430-22(a). This results in a minimum conductor ampacity of (1.25 x 20) 25. The ampacity of No. 12 copper wire is listed at 25 in Table 310-16, and Section 240-3(g) indicates that the restriction on the rating of the overcurrent device as outlined in Section 240-3(d) does not apply to motor circuits. Therefore, Number 12 copper wire protected by a 50-ampere, single-pole circuit breaker meets the requirements in Section 430-52 (c)(1) and Table 430-152. One reason for limiting the ampere rating of overcurrent devices to 15, 20, and 30 for Nos. 14, 12, and 10 copper conductors was to avoid having to retest and, in some cases, redesign 15- and 20-ampere wiring devices, terminals on overcurrent devices, and all related electrical equipment that has been tested with No. 12 copper wire and 20 ampere overcurrent devices. Although the ampere ratings of overcurrent devices for these small circuit conductors are limited, the ampacities shown in Table 310-16 may be used for derating because of elevated ambient temperatures or where there are more than three current carrying conductors in a raceway or cable. Section 310-15(b) indicates that conductor ampacities given in Table 310-16 are permitted where adjustment factors are used because there are more than three current carrying conductors in a raceway or cable. Conductor ampacities given in Table 310-16 are also used where it is necessary to apply a correction factor because of elevated temperature. Although the higher ampacity in the Table can be used for derating, the overcurrent protection mentioned in Section 240-3(d) cannot be exceeded although the final calculated ampacity is greater. Transfer Switches Connected to Emergency Generators Q: Is it permissible to connect an electrically operated and electrically held transfer switch that supplies nonemergency loads to an emergency generator? This transfer switch would supply a battery charger for an uninterrupted power supply (UPS) system that is connected to computers. A: The requirement for an electrically operated and mechanically held transfer switch for emergency systems is new in the 1999 edition of the NEC. Although Underwriters Laboratories Inc. (UL) lists only electrically operated, mechanically held transfer switches for emergency systems, enforcing authorities have accepted electrically operated, electrically held transfer switches for this duty. Part of Section 700-6 (a) requires that automatic transfer switches be identified for such use, and approved by the authority having jurisdiction. The definition for "identified" appears in Article 100 and reads like this: "Recognizable as suitable for the specific purpose, function, use, environment, application, etc., where described in a particular Code requirement." A Fine Print Note explains that a qualified testing laboratory, inspection agency, or other organization concerned with product evaluation may determine suitability of equipment for a specific purpose. The transfer switch that supplies emergency loads cannot supply anything else. (See Section 700-6[d]). Therefore, a manual or automatic transfer switch must be provided to supply the optional standby load. An electrically operated, electrically held transfer switch may be used for this purpose. If the generator is not large enough to supply the emergency load and all of the optional standby loads at the same time, the optional standby loads may have to be cycled on and off to prevent overloading the generator. This practice is permitted by Section 700-5( a ) and (b). Sizing Equipment Grounding Conductors for Taps Q: How do you size the equipment grounding conductor for a tap? We have an existing 150-ampere feeder, three-wire single phase that will be tapped with No. 8 Type THWN copper conductors. The length of the tap conductors will be approximately 21 feet. The wiring method is rigid nonmetallic conduit. The tap conductors will terminate in a 40-ampere overcurrent device. Is the equipment grounding conductor size based on the 150-ampere overcurrent device protecting the feeder or the 40-ampere overcurrent device at the load end of the tap? A: Section 240-21(b)(2) recognizes a tap not over 25 feet long where these three conditions are met: (1) The ampacity of the tap conductors is not less than one-third the rating of the overcurrent device protecting the feeder conductors; (2) the tap conductors terminate in a single overcurrent device that will limit the load to the ampacity of the tap conductors; and (3) the tap conductors are protected from physical damage or are installed in a raceway. Item (1) is satisfied because No. 8 Type THWN copper wire has an ampacity of 50, which is exactly one-third of 150. Item (2) is satisfied because the single overcurrent device at the load end of the tap is a 40 amperes. The ampere rating of this overcurrent device matches the ampacity of a No. 8 copper conductor with 60 degrees C insulation and complies with the temperature limitations placed on terminations as outlined in Section 110-14( c )(1). I assume that item (3) is satisfied by installing the rigid nonmetallic conduit in accordance with Article 347. The equipment grounding conductor size is based on the ampere rating (150) of the feeder. Table 250-122 lists the minimum sizes for equipment grounding conductors based on the ampere ratings of the overcurrent devices. Moreover, for a 150-ampere overcurrent device, the minimum size equipment grounding conductor shown in the Table is No. 6 copper. However, part (a) of Section 250-122 points out that the equipment grounding conductor is not required to be larger than the circuit conductors are. Therefore, the equipment grounding conductor in the rigid nonmetallic conduit cannot be smaller than No. 8 copper. FLACH, a regular contributing Code editor, is a former chief electrical inspector for New Orleans. He can be reached at (504) 254-2132.

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